The present invention relates generally to an apparatus for in vivo imaging. More particularly, the present invention relates to a catheter that incorporates an Optical Coherence Tomography (OCT) system and an Intravascular Ultrasound (IVUS) system for concurrent imaging of luminal systems, such as imaging the vasculature system, including, without limitation, cardiac vasculature, peripheral vasculature and neural vasculature.
Myocardial infarction or heart attack remains the leading cause of death in society. Until recently, many investigators believed that the primary mechanism for myocardial infarction was coronary arteries critically blocked with atherosclerotic plaque that subsequently progressed to total occlusion. Recent evidence from many investigational studies, however, clearly indicates that most infarctions are caused by sudden rupture of non-critically stenosed coronary arteries resulting from sudden plaque rupture. For example, Little et al. (Little, W C, Downes, T R, Applegate, R J. The underlying coronary lesion in myocardial infarction: implications for coronary angiography. Clin Cardiol 1991, 14: 868-874, incorporated by reference herein) observed that approximately 70% of patients suffering from an acute plaque rupture were initiated on plaques that were less than 50% occluded as revealed by previous coronary angiography. This and similar observations have been confirmed by other investigators (Nissen, S. Coronary angiography and intravascular ultrasound. Am J Cardiol 2001, 87 (suppl): 15A-20 A, incorporated by reference herein).
The development of technologies to identify these unstable plaques holds the potential to decrease substantially the incidence of acute coronary syndromes that often lead to premature death. Unfortunately, no methods are currently available to the cardiologist that may be applied to specify which coronary plaques are vulnerable and thus prone to rupture. Although treadmill testing has been used for decades to identify patients at greater cardiovascular risk, this approach does not have the specificity to differentiate between stable and vulnerable plaques that are prone to rupture and frequently result in myocardial infarction. Inasmuch as a great deal of information exists regarding the pathology of unstable plaques (determined at autopsy), technologies based upon identifying the well-described pathologic appearance of the vulnerable plaque offers a promising long term strategy to solve this problem.
The unstable plaque was first identified and characterized by pathologists in the early 1980's. Davis noted that with the reconstruction of serial histological sections in patients with acute myocardial infarctions associated with death, a rupture or fissuring of athermanous plaque was evident (Davis M J, Thomas A C. Plaque fissuring: the cause of acute myocardial infarction, sudden death, and crescendo angina. Br Heart J 1985; 53: 3 63-37 3, incorporated by reference herein). Ulcerated plaques were further characterized as having a thin fibrous cap, increased macrophages with decreased smooth muscle cells and an increased lipid core when compared to non-ulcerated atherosclerotic plaques in human aortas (Davis M J, Richardson E D, Woolf N. Katz O R, Mann J. Risk of thrombosis in human atherosclerotic plaques: role of extracellular lipid, macrophage, and smooth muscle cell content, incorporated by reference herein). Furthermore, no correlation in size of lipid pool and percent stenosis was observed when imaging by coronary angiography. In fact, most cardiologists agree that unstable plaques progress to more stenotic yet stable plaques through progression via rupture with the formation of a mural thrombus and plaque remodeling, but without complete luminal occlusion (Topol E J, Rabbaic R. Strategies to achieve coronary arterial plaque stabilization. Cardiovasc Res 1999; 41: 402-417, incorporated by reference herein). Neovascularization with intra-plaque hemorrhage may also play a role in this progression from small lesions, i.e., those less than about 50% occluded, to larger significant plaques. Yet, if the unique features of unstable plaque could be recognized by the cardiologist and then stabilized, a dramatic decrease may be realized in both acute myocardial infarction and unstable angina syndromes, and in the sudden progression of coronary artery disease.
The present invention uses depth-resolved light reflection or Optical Coherence Tomography to identify the pathological features that have been identified in the vulnerable plaque. In OCT, light from a broad band light source or tunable laser source is split by an optical fiber splitter with one fiber directing light to the vessel wall and the other fiber directing light to a reference mirror. The distal end of the optical fiber is interfaced with a catheter for interrogation of the coronary artery during a heart catheterization procedure. The reflected light from the plaque is recombined with the signal from the reference mirror forming interference fringes (measured by a photovoltaic detector) allowing precise depth-resolved imaging of the plaque on a micron scale.
OCT uses a superluminescent diode source or tunable laser source emitting a 400-2000 nm wavelength, with a 50-250 nm band width (distribution of wave length) to make in situ tomographic images with axial resolution of 2-20 μm and tissue penetration of 2-3 mm. OCT has the potential to image tissues at the level of a single cell. In fact, the inventors have recently utilized broader bandwidth optical sources so that axial resolution is improved to 4 um or less. With such resolution, OCT can be applied to visualize intimal caps, their thickness, and details of structure including fissures, the size and extent of the underlying lipid pool and the presence of inflammatory cells. Moreover, near infrared light sources used in OCT instrumentation can penetrate into heavily calcified tissue regions characteristic of advanced coronary artery disease. With cellular resolution, application of OCT may be used to identify other details of the vulnerable plaque such as infiltration of monocytes and macrophages. In short, application of OCT can provide detailed images of a pathologic specimen without cutting or disturbing the tissue.
An OCT catheter to image coronary plaques has been built and is currently being tested by investigators. (Jang I K, Bouma B E, Hang O H, et al. Visualization of coronary atherosclerotic plaques in patients using optical coherence tomography: comparison with intravascular ultrasound. JACC 2002; 3 9: 604-609, incorporated by reference herein). The prototype catheter consists of a single light source and is able to image over a 360 degree arc of a coronary arterial lumen by rotating a shaft that spins the optical fiber. Because the rotating shaft is housed outside of the body, the spinning rod in the catheter must rotate with uniform angular velocity so that the light can be focused for equal intervals of time on each angular segment of the coronary artery.
While OCT imaging provides high resolution (2-20 μm) tomographic visualization of coronary arteries, OCT, however, lacks penetration with a maximum penetration depth of only 2-3 mm into the tissue. The present invention overcomes this disadvantage by incorporating an ultrasound transducer suitable for performing intravascular ultrasound (“IVUS”) into an OCT catheter to form an OCT-IVUS catheter. The present invention uses IVUS imaging to identify the pathological features that have been identified in the vulnerable plaque. A particularly valuable tool, IVUS technology uses high frequency sound waves to detect blood vessel blockages and other problems such as aneurysms.
Ultrasound imaging systems can be equipped with a 38 mm aperture, broadband (5-10 MHz) linear array transducer. Cells can be imaged in color power Doppler, power Doppler, M-mode and B-scan modes. B-scan sonogram images, also called the grayscale mode, are the typical ultrasound method to monitor or examine the human body using backscattering of acoustic waves. M-mode ultrasound employs a sequence of scans at a fixed ultrasound beam over a given time period. M-mode is used for visualizing rapidly moving subjects, such as heart valves. Compared to conventional B-scan images, Doppler ultrasound is used to assess changes in the frequency of reflected acoustic waves. Color power Doppler converts reflected acoustic waves that are Doppler shifted into colors that overlay the conventional B-scan images and can indicate the speed and direction of moving objects. Power Doppler ultrasound is most commonly used to evaluate moving objects and has higher sensitivity than the color power Doppler mode. The gain of the color power and Doppler imaging mode can be manually adjusted to suppress the background noise. If the settings of the ultrasound instrumentation remain unchanged, objective comparisons of each can be made. Additional disclosure explaining how OCT and ultrasound imaging systems work and cooperate can be found in commonly assigned and co-pending U.S. patent application Ser. No. 11/550,771, filed Oct. 18, 2006 and published as U.S. Publication No. US 2008-0097194 on Apr. 24, 2008, which is incorporated herein by reference in its entirety.
IVUS is a widely available clinical tool for guiding percutaneous interventions and/or intraluminal imaging. While IVUS uses frequencies from 20 to 40 MHz and provides good depth penetration, it lacks sufficient resolution (˜120 μm) to study thin-cap thrombus or atheroma lesions and other fine details with the vasculature. Conversely, while OCT provides high resolution (2-20 μm) tomographic visualization of coronary arteries, OCT, however, lacks penetration with a maximum penetration depth of only 2-3 mm. However, it has been found that OCT can image behind calcifications clearly while ultrasounds are intensely reflected. The current high resolution capabilities of OCT are well suited for imaging vulnerable plaques but poor depth penetration hamper full characterization of coronary lesions and plaque burden. Because IVUS penetrates deeper into the media and adventitia, combining OCT and IVUS modalities will enhance quantitative analysis of coronary arteries significantly.